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Why do realists use the history of atoms to argue for the existence of unobservable entities?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — Philosophy of science Aristotle, marble portrait bust, Roman copy (2nd century bce) of a Greek original (c. 325 bce); in the Museo Nazionale Romano, Rome.

In the philosophy of science, realists use the history of atoms to argue for the existence of unobservable entities as part of what is often called the "ultimate argument for realism."1

This argument, notably developed by Wesley Salmon, focuses on the work of the French physicist Jean Perrin. Salmon observed that scientists arrived at the same values for atomic quantities through many independent experimental methods. Realists argue that it would be an extraordinary coincidence—a "miracle"—if these unobservable entities did not actually exist, given that these diverse, independent lines of inquiry consistently converged on the same results.1

By demonstrating that systematic success in predicting observable phenomena is best explained by the truth of the underlying theoretical claims, realists contend that we have strong grounds to believe in the reality of these unobservable entities. They argue that it would be a form of metaphysical hubris to assume that the connection between success and truth breaks down simply because the entities involved lie below the threshold of human observation.12